Most turbomachinery reaches very high rotational speeds, for example from 20,000 to 120,000 revolutions per minute (rpm). To guide a rotor rotating at such speeds, it is conventional to use foil bearings instead of conventional ball bearings which are not compatible with such rotational speeds. A film of ambient air is used to support a rotor which is thus “levitated” relative to a fixed housing. Such a bearing is therefore also called a foil-air bearing.
When the rotor rotates at high speed, there is no contact between the various solid components of the bearing. However, before and after a phase of high-speed rotation, at startup or shutdown, the film of air which serves as a bearing has not been formed in the stationary housing, and a foil, called the top foil, of the foil bearing is in local contact with both the rotor and the fixed housing that receives them. Means to limit friction between these elements should then be provided at the contact surfaces.
It is known to use PTFE (Polytetrafluoroethylene) as a lubricant to prevent wear of the parts in a foil bearing during the startup and shutdown phases. This material is usually covering a foil of the bearing which faces the rotating part, also called the top foil, and has good performance at relatively low temperatures but cannot be used at high temperatures (above 200° C. or so).
When the rotor is part of a machine with a relatively powerful electric motor, the motor is a source of heat. The ambient air used for ventilation in this machine is then hot air. Due to the heat of the motor plus the heat locally generated by friction, insufficient heat dissipation may occur and high temperatures of around 300 to 400° C. can be reached.
U.S. Pat. No. 5,866,518 proposes a composite self-lubricating material to reduce friction and wear which can be used over a very wide temperature range, from cryogenic temperatures to temperatures up to 900° C. This material comprises 60 to 80 wt % (percentage by weight) of chromium oxide dispersed in a metal binder of an alloy containing chromium and possibly nickel as well as 5 to 20 wt % of a fluoride from groups I or II, or a rare earth metal and possibly a metal lubricant.
U.S. Pat. No. 7,297,367 relates to a method for applying an inorganic lubricant coating deposited on a top foil of a foil bearing in order to withstand higher temperatures than inorganic coatings of the prior art. The coating is applied by spraying or by immersion.
The present invention therefore aims to provide a new coating to protect the foil(s) of a foil bearing from wear and to withstand high temperatures.